This invention relates to the production of flexible electrochromic devices using electrically conductive flexible substrates and common manufacturing techniques.
An electrochromic material undergoes a reversible color change upon the adsorption and desorption of small cations. This property can be exploited to fabricate a device that changes color upon the application of a voltage potential.
A typical electrochromic device comprises an electrochromic layer and an ion storage layer sandwiched between two conducting substrates, at least one of which is transparent. Optionally, the electrochromic layer and the ion storage layer can be separated by an ion-conducting electrolyte layer. Optical properties of the electrochromic device change when ions (e.g., hydrogen ions or lithium ions) intercalated within the structure of the ion-storage layer are removed and interposed within the structure of the electrochromic material in response to an electric potential applied to the conductive substrates. Ions are removed and returned to the ion-storage layer by reversing the polarity of the applied potential, thereby returning the electrochromic device to its original optical state.
The electrochromic layer and the ion storage layer are similar in that they both adsorb and desorb mobile ions in response to an applied electric field. A simple model for understanding electrochromic devices assumes that the electrochromic layer colors and clears during ion adsorption/desorption, while the ion storage layer remains transparent. However, practical electrochromic devices can be made if the ion storage layer colors as well. For example, if the electrochromic layer cycles from clear to color upon ion adsorption (e.g. tungsten oxide), and the ion storage layer cycles from clear to color upon ion desorption (e.g. nickel oxide), the overall devices will cycle from clear to color. If the electrochromic layer cycles from clear to blue upon ion adsorption (e.g. tungsten oxide), and the ion storage layer cycles from clear to yellow upon ion adsorption (e.g. vanadium oxide), the overall device will cycle from blue to yellow. Numerous combinations are possible.
Furthermore, if the ion-conducting electrolyte layer is opaque and the electrochromic layer cycles from clear to blue, the entire device will cycle from blue to the color of the ion-conducting electrolyte layer, regardless of the coloration of the ion storage layer.
The construction of an electrochromic device typically involves coating electrochromic material onto a transparent, conductive substrate. If the transparent, conductive substrate comprises glass, there are several proven coating methods available. These include evaporation deposition (Green, U.S. Pat. No. 5,598,293) and electro-deposition (Tseung et. al., U.S Pat. No. 5,470,673). Of particular advantage and commercially available utility is the coating of a transition metal alkoxide from an alcoholic solution (Moser et al., U.S. Pat. No. 4,855,161), followed by heating in excess of 200xc2x0 C. Processes involving the coating from alcoholic solutions feature good uniform coverage, low capital investment, and low chemical toxicity.
An electrochromic device comprising flexible plastic substrates would have advantages over electrochromic devices comprising glass substrates. These advantages include light weight, durability, shapability and low cost. However, the known processes of coating from alcoholic solutions fail on commercially available electrically conductive plastic substrates. First, plastic substrates cannot tolerate high processing temperatures without serious degradation. Second, commercially available plastic substrates with conductive coatings often have a thin non-conductive layer applied to the conductive layer to promote conductor adhesion to the plastic substrate and to protect the conductive layer against scratching. This anti-scratching coating can cause poor adhesion of the electrochromic material deposited from alcoholic coating solutions. This is demonstrated in Examples 1, 2, and 4 below.
In U.S. Pat. No. 5,471,554, Rukavina et al. eliminate the anti-scratching coating by providing an adhesion layer between the plastic substrate and the transparent conductive layer. Yu et al., in U.S. Pat. No. 5,471,338 disclose similar technology. This is a relatively protracted process that still leaves the conductive layer subject to scratching during storage and subsequent processing.
In U.S. Pat. No. 5,812,300, Coleman describes an electrochromic device formed on a polyethylene terephthalate substrate by printing a silver flake/resin conductive layer and then an electrochrorniic layer coating from toluene. The silver flake conductive layer is not transparent, thus severely limiting the utility of the device. Furthermore, toluene is a harsh solvent that can deteriorate the optical clarity of the plastic conductive substrate.
In U.S. Pat. No. 5,277,986, Cronin et al. describe the incorporation of a fugitive organic material within the ethanolic coating solution to provide porosity of the electrochromic layer. This process requires temperatures in excess of that which plastic substrates can tolerate.
In U.S. Pat. No. 5,124,080, Shabrang et al. describe the use of perfluorosulfonic acid polymer as an ion-conducting electrolyte interposed between an electrochromic layer and an ion storage layer. Shabrang does not anticipate that incorporating perfluorosulfonic acid polymer into the electrochromic layer or the ion storage layer would be particularly advantageous for coating on plastic substrates.
In U.S. Pat. No. 5,825,526, Bommarito et al. describes vacuum deposition of electrochromic metal oxides on flexible transparent substrates, followed by a topcoat comprising a perfluoroamide. Forming an electrochromic layer of sufficient porosity and thickness on plastic substrates by means of vacuum deposition is difficult and expensive.
Glass electrochromic devices have not achieved broad commercial acceptance in architectural, automotive or eyewear applications, due to practical limitations. First, glass electrochromic devices can be prohibitively expensive to manufacture. Second, glass electrochromic devices cannot function the decades required for architectural and automotive applications. With each cycle, an electrochromic device suffers a minute but cumulative deterioration in performance, due to the accumulation of an irreversible colored xe2x80x9cbronzexe2x80x9d and trapped gas. Third, glass electrochromic devices are too heavy for eyewear applications and can also shatter to dangerous shards upon impact of a foreign object.
Plastic electrochromic devices address these limitations. Manufacturing costs are controlled by low capital requirements and high throughput. For example, in accordance with the present invention, an electrochromic layer or an ion storage layer could be coated on a continuous wide web of electrically conductive polyethylene terephthalate film at a rapid rate. The layers could then be laminated together using an adhesive ion conducting electrolyte. If this laminated film is applied to architectural and automotive glazing, the film could be replaced if its performance deteriorates over time. If the laminated film is applied to polycarbonate, or if the electrochromic device included polycarbonate coated directly, the electrochromic device would be light and safe enough for eyewear.
The present invention provides a composition of an electrochromic layer which coats uniformly from an alcoholic solution onto electrically conductive, flexible plastic, metal or fabric substrates which require low processing temperatures. This composition comprises a perfluorosulfonated anionic polyelectrolyte and a metal oxide selected from the group consisting of tungsten oxide, molybdenum oxide, niobium oxide, vanadium oxide, nickel oxide, cerium oxid titanium oxide, copper oxide, chromium oxide, rhodium oxide, manganese oxide, ruthenium hydroxide, osmium hydroxide, iridium oxide and mixtures thereof.
The present invention further provides a composition of an ion storage layer which coats uniformly from an alcoholic solution onto electrically conductive, flexible plastic, metal or fabric substrates and which require low processing temperatures. This composition comprises a perfluorosulfonated anionic polyelectrolyte and a metal oxide selected from the group consisting of tungsten oxide, molybdenum oxide, niobium oxide, vanadium oxide, nickel oxide, cerium oxide, titanium oxide, copper oxide, chromium oxide, rhodium oxide, manganese oxide, ruthenium hydroxide, osmium hydroxide, iridium oxide and mixtures thereof.
The metal substrate may be made of any electrically conductive metal or metal alloy, such as stainless steel, steel, nickel, aluminum, iron, copper, gold, silver, platinum, palladium, indium, tin and chromium.
The present invention further provides the means to fabricate electrochromic devices with the advantages of light weight, durability and flexibility.